In pulse-echo nondestructive testing employing ultrasonic wavetrains, resolution of defects and penetration of material are known to be inherently conflicting requirements. Yet, typical test aplications do involve simultaneous detection of defects close to the sound-beam entry surface as well as deep within the test object.
Near-surface resolution is usually achieved by the use of short wavetrains having only one or two cycles of the nominal test frequency and a correspondingly high harmonic content. Amplification of these without distortion and undesirable pulse stretching requires wideband amplifiers which tend to produce high background noise, particularly at the very high gains needed to detect small defects through the full cross section of typical test samples. Conversely, deep penetration of material is best obtained using relatively long wavetrains having several cycles of carrier and negligible higher order harmonic content. These wavetrains are compatible with tuned amplifiers of limited bandwidth which can have very high gain without excessive noise. As a result of these conflicting requirements, typical prior art instrumentation for ultrasonic flaw detection has involved compromises in amplifier design which limit optimum performance as discussed in the text by Krautkramer, ULTRASONIC TESTING OF MATERIALS, Springer-Verlag, N.Y., 1969, pg, 165-166.
Various solutions have been proposed heretofore for optimizing test results, exemplified as follows. Weighart in U.S. Pat. No. 3,033,029 describes a distance-amplitude compensation system based on time-varied receiver gain. Weighart in U.S. Pat. No. 3,309,914 proposes the use of a multi-frequency testing system to achieve both resolution and penetration. Couture in U.S. Pat. No. 3,823,603 employs a gated attenuator at the amplifier input to change system sensitivity abruptly. McElroy, et al, in U.S. Pat. No. 3,924,454 also use a multiple frequency test system with the addition of a search unit having a multi-element transducer.
In accordance with the present invention, performance is optimized by varying receiver-amplifier bandwidth as a function of time.